1. Field of the Invention
The present invention relates to a degradable monofilament which has an excellent linear tensile strength, flexibility and moderate degradability at the same time and is excellent in ligature stability (or knot security); a preparation process of the same; and a medical device prepared from said degradable monofilament.
2. Description of the Related Art
Conventionally in a medical field such as surgery, a synthetic absorbable surgical suture has been widely used. Polyglycolic acid (PGA), polylactic acid (PLA) and a glycolic acid/lactic acid copolymer (PLGA) are used for a raw material of the suture. PGA, PLA and PLGA are bioabsorbable polymers and can be degraded in a living body to form degraded products such as lactic acid and glycolic acid. These acids are finally decomposed through a metabolic pathway into carbon dioxide and water, and externally released from the living body. PGA, PLA and PLGA are individually prepared by polymerization of glycolide (GLD) which is a cyclic dimer of glycolic acid, polymerization of lactide (LTD) which is a cyclic dimer of lactic acid, and ring-opening copolymerization of a mixture of GLD and LTD.
PGA and other bioabsorbable polyesters have good processing ability and can provide a high strength filament. On the other hand, the filament has high rigidity and thus a thick monofilament such as a fishing line is very difficult to carry out ligation and is unsuitable for the surgical suture.
Consequently, in the case of preparing the synthetic absorbable surgical suture from PGA or PLGA, many fine filaments are usually spun in order to obtain flexibility, braided, and used as a so-called multifilament. However, the multifilament suture has an irregular surface and leads to a problem of wounding a surrounding tissue of the living body in the course of suture. Further, application of a coating agent is required in order to make the filament slippery in the ligation. Such a treatment renders a manufacturing step complex and economically unfavorable.
In recent years, several monofilament absorbable sutures have been developed in order to overcome the problems noted above.
For example, U.S. Pat. No. 4,700,704 has disclosed a sterilized surgical product which has a sequence on the basis of about 20 to 35 wt % .epsilon.-caprolactone(CL) and about 65 to 80 wt % GLD, consists of a polymer material having a melting point of 213.degree. C. or less, and has a tensile strength of 30,000 psi or more and a young's modulus of less than 350,000 psi.
Further, a surgical monofilament suture prepared from a copolymer of about 25 mol % CL and about 75 mol % GLD has been described in Biomaterials, Vol. 16, No. 15, pp 1141-1148(1995).
The surgical devices which have been disclosed in these preceding references and prepared from a copolymer of GLD and CL have an excellent flexibility and mechanical strength and thus have an advantage that it is possible to utilize for a surgical suture in the form of monofilament. However, these surgical devices have too high velocity of hydrolysis and quickly decompose in a living body. As a result, it is unsatisfactory to use for a surgical suture and ligation material in the operation of diseased parts having a long recovery period.
Further, the filament above is difficult to make a small and stable knot when ligating with a surgeon's knot in the suture. Thus, the knot is liable to be large and the sutured part tends to become loose. Such a phenomenon is extremely unfavorable in view of the essential object for use of the suture. In order to obtain reliability and safety of ligature, surgeons were required to stabilize the suture by making many knots on the suture.
Japanese Laid-Open Patent HEI 1-175855 has disclosed a stretched surgical monofilament suture consisting of polycaprolactone (PCL) having a relative a viscosity of 2.0 to 8.2. However, degradation and absorption of the PCL suture are very slow in a living body and thus PCL is not practical for an absorbable suture.
Further, U.S. Pat. No. 5,252,701 has disclosed a segmented copolymer having two or more species of different ester bond and exhibiting absorption ability into a living body. The patent has disclosed a segmented copolymer comprising a plurality of fast transesterifying linkages which substantially consists of a glycolate linkage and a plurality of slow transesterifying linkages which is selected from the group consisting of trimethylene carbonate linkages and caproate linkages. The patent has also disclosed a preparation process of the above segmented copolymer wherein two or more species of different cyclic monomers are successively added in two or more steps, a molten copolymer is formed, and the copolymer is further heated to undergo transesterification. The segmented copolymer markedly differs in physical properties from a random copolymer or block copolymer.
However, as shown in the patent specification, the greater the progress of segmentation (transesterification or reshuffling) in the segmented copolymer, the higher the reduction of the melting point and crystallinity of the copolymer. Thus, according to the information of the present inventors, the monofilament prepared from the segmented copolymer does not exhibit sufficient tensile strength for the suture. Similarly, according to the information of the present inventors, low crystallinity of the segmented copolymer leads to too rapid velocity of hydrolysis in a living body. Thus, the segmented copolymer is unsatisfactory to the suture or ligation material for operation of diseased parts requiring a long recovery period.
On the other hand, Japanese Laid-Open Patent HEI 6-16792 has disclosed a preparation process of an CL/GLD copolymer comprising using a polymerization reactor which is equipped with a raw material supply port, copolymer discharge port, exhaust port and stirrer which has excellent surface renewability and is used for the high viscosity solution, forming a low molecular weight oligomer of CL at temperature of 250.degree. C. or less in the first step, adding GLD in the second step and polymerizing at temperature of 100.degree. C. or more, terminating the polymerization reaction by addition of acid anhydride or acid halogenide to the resulting copolymer in the molten state, and successively removing the unreacted monomer with stirring under reduced pressure. However, acid anhydride and acid halide develop peculiar irritating smell even in a trace amount and thus specific design is required for the equipment in the manufacturing factory. Further, it is difficult to completely remove unreacted acid anhydride and acid halogenide from the copolymer, and the resulting copolymer and suture have a disadvantage in smell. The patent specification has no disclosure at all upon the monofilament having flexibility and high strength and the preparation process of the same.
Consequently, one object of the invention is to provide a degradable monofilament which has an outstanding mechanical strength and flexibility, is excellent in ligation stability and has a moderate velocity of hydrolysis, and is suitable for a material of an absorbable surgical suture.